The invention concerns a measuring device for nuclear magnetic resonance (NMR) measurements having an NMR probe head containing one or more NMR receiver coils. The probe head can be supplied with coolant from a cooling device via a cryogenically insulated transfer conduit, wherein the cooling device comprises a cryo-cooler having a first cooling stage with a first stage exchanger and with a second cooling stage having a second stage exchanger, wherein a pump is provided for transporting the coolant, initially at room temperature, within a circuit through a first counter-flow exchanger, the first stage exchanger, a second counter-flow exchanger, the second stage exchanger and the transfer conduit into the NMR probe head for cooling the NMR receiver coil(s) and back again through the transfer conduit, the second counter-flow exchanger and the first counter-flow exchanger. NMR measuring device of this kind is known in the art from U.S. Pat. No. 5,508,613.
NMR spectroscopy is a measuring method with which the structure of chemical compounds can be precisely determined. This advantage has, however, a greater associated disadvantage: NMR spectroscopy is unfortunately also a relatively insensitive measurement method which, in general, only has rather small signal-to-noise-ratios (S/N-ratio). It is therefore necessary to entertain all possible steps which could lead to an increase in sensitivity.
This S/N-ratio can be substantially improved by optimizing the geometry, through careful choice of the materials and by using precisely tuned material compositions for the NMR receiver coil. However, it turns out that one has now reached the limits of this type of optimization process and further improvements can only be expected in small steps. For this reason, it has become necessary to investigate new methods for optimizing even if same are associated with substantial effort and expense. One such possibility is the cryogenic cooling of the receiver coil with its electrical resonant and tuning network. In an additional step, the preamplifier must also be cooled so that its noise remains small relative to the receiver coil.
The expression "receiver coil" is not meant to refer only to pure inductances but also to resonators comprising distributed inductance and/or capacitance to effect a system capable of resonance in the radio frequency region.
In most NMR measurements, the S/N-ratio of the NMR signal at the output of the receiver is primarily limited by the S/N ratio of the receiver coil. This S/N ratio depends on the size of the NMR signal received from a standard sample compared to the intrinsic noise of the coil. The size of the NMR signal depends on the geometry of the receiver coil and on how closely the coil surrounds the sample. These properties cannot be influenced by temperature. This is not the case, however, for the intrinsic noise of the coil which is produced by the high-frequency loss resistance R.sub.HF of the coil and depends on the size of its resistance R.sub.HF and on its temperature: more precisely, on the square root of the product R.sub.HF .multidot.T.
Cooling the receiver coil to below 30 K. reduces both its resistance R.sub.HF as well as its temperature T leading to a substantial reduction in the intrinsic noise and to a corresponding increase in the S/N ratio.
U.S. Pat. No. 5,508,613 discloses a conventional NMR measuring device having a cooled NMR receiver coil. An additional problem not solved by the conventional apparatus is, however, the intrinsic noise of the preamplifier which amplifies the NMR signals emanating from the NMR receiver coil and which normally operates at room temperature.
It is therefore the purpose of the present invention to present an NMR measuring device of the above mentioned kind with which a substantial reduction in noise, including that of the preamplifier, is effected in as simple a manner as possible and without substantial technical difficulty and expense.